feat(web): migrate to Next.js App Router and add design tokens (ADR 0018)

The Sprint 1 shell moves to /app with its own root layout so its dark theme does not leak. Figma tokens ported with light and dark parity.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014SdXCbrN5PtGExkpvMaMHx

web/package.json

@@ -4,11 +4,14 @@
   "type": "module",
   "scripts": {
     "test": "vitest run",
-    "dev": "vite",
-    "build": "tsc -b && vite build",
-    "preview": "vite preview"
+    "dev": "next dev -p 5173",
+    "build": "next build",
+    "start": "next start -p 5173",
+    "lint": "tsc --noEmit"
   },
   "dependencies": {
+    "geist": "^1.7.2",
+    "next": "^16.4.0",
     "react": "^19.2.8",
     "react-dom": "^19.2.8"
   },